A light having a wavelength of 1 to 10 nanometers is called soft X-ray. This soft X-ray is used for a microscope used for observing a fine structure of living cells. Also, EUV having a wavelength of 10 to 20 nanometers is used for next-generation lithography. In particular, soft X-ray having a wavelength of 2 to 4 nanometers is very suitably used for a microscope. It is because the soft X-ray exhibits a great transmittance difference for protein and water in the wavelength region of 2 to 4 nanometers (hereinafter, referred to as “water window” region). Namely, the soft X-ray in the water window region has good transmittance for water but bad transmittance for protein, so it is a very suitable light source for investigating a cell interior structure.
Strong soft X-ray is generated in synchrotron facilities. However, the synchrotron facilities are very huge experiment devices, which consume a lot of time and cost. Thus, the synchrotron facilities are not suitable for being used in a small laboratory.
In a small laboratory, a laser plasma light source generator is used as a light source generator. The laser plasma light source generator irradiates a high power laser beam to a target arranged in a vacuum container to generate a light. If a high power laser beam is concentrated on the target, high density plasma is produced. The produced plasma is freely expanded in the vacuum container, and the light is generated from the expanded plasma.
The laser plasma light source generator can generate a light with a wavelength in X-ray and EUV regions depending on the target. For example, a light with a wavelength of 13.5 nanometers for extreme ultraviolet lithography can be generated when a material such as xenon (Xe), lithium (Li) and tin (Sn) is used as a target, and a light in the water window region can be generated when a material including nitrogen (N) atom or carbon (C) atom is used.
However, in case a solid target composed of the above atoms is used, if the density of nitrogen atoms or carbon atoms in the solid target is not high, the intensity of generated light is relatively low.
Also, if a high power laser is irradiated to only a certain portion of a solid target, the laser concentrated area is deformed. Thus, it is required to rotate or vertically/horizontally move the solid target such as the laser may be always input to a new area, which however needs a driving mechanism. Also, the solid target should be exchanged after once used. Thus, there is a lot of inconvenience in use, and a lot of time and cost is consumed.
In addition, if a solid target is used, scattering particles (hereinafter, referred to as “debris”) such as pyrolysate or chips are emitted from the solid target together with the soft X-ray and EUV. This debris is scattered and floating in all directions. In particular, the laser beam has a high out-put, the debris has a very increased speed. Such debris may damage expensive surrounding optical devices, which is considered as the most serious problem.
In order to solve this problem, the applicant filed a patent application in Korea, entitled “an X-ray and EUV light source generator using a liquid target”, on Sep. 23, 2005, which is registered (Registration No. 0617603).
However, according to the light source generator disclosed in the Korean Patent Registration No. 0617603, a liquid target supplier having only one capillary tube is coupled to a vacuum chamber. Thus, first, in case the capillary tube is damaged, the capillary tube should be exchanged, which demands much time. Second, when it is intended to inject another kind of liquid target from the capillary tube, a gas supplied to the capillary tube should be exchanged.
In addition, according to the light source generator disclosed in the Korean Patent Registration No. 0617603, it is impossible to control temperature of a cooling solvent used for liquefying the supplied gas. Thus, first, the kind of supplied gas is limited to one having a higher liquefaction temperature than the temperature of the cooling solvent. Second, when it is intended to use a gas with a lower liquefaction temperature than the temperature of the coolant solvent for forming a liquid target, the cooling solvent should be exchanged with a cooling solvent with a lower temperature than the liquefaction temperature of the gas.